Background: Mifepristone is a highly effective and well-tolerated emergency contraceptive when given in a dose of 600 mg within 72 h of unprotected coitus. We assessed whether the same effectiveness can be achieved with lower doses of mifepristone (50 mg and 10 mg) and a longer postcoital treatment period (120 h).
Methods: We undertook a multicentre, single-masked, randomised trial in 11 family-planning clinics in Australia, China, Finland, Georgia, the UK, and the USA. 1717 healthy women with regular menstrual cycles who requested emergency contraception within 120 h of unprotected coitus were randomly assigned to three treatment groups.
Findings: 32 women were lost to follow-up and one was pregnant before treatment. The 600 mg, 50 mg, and 10 mg groups did not differ in the proportions of pregnancies (seven [1.3%] of 559, six [1.1%] of 560, and seven [1.2%] of 565). Two pregnancies (both in the 50 mg group) were tubal. Among women without further acts of intercourse, treatment delay did not appear to influence the effectiveness. No major side-effects occurred, except a delay in the onset of next menses, significantly (p<.001) related to the mifepristone dose.
Interpretation: Lowering the dose of mifepristone sixty-fold did not decrease its effectiveness as an emergency contraceptive under typical use, though a study of this size cannot exclude differences in effectiveness up to almost three-fold. Lower doses of mifepristone were associated with less disturbance of the menstrual cycle. Thus, a dose as low as 10 mg seems preferable to the 600 mg dose.